470 research outputs found
Extreme Precision Antenna Reflector Study Results
Thermal and mechanical distortion degrade the RF performance of antennas. The complexity of future communications antennas requires accurate, dimensionally stable antenna reflectors and structures built from materials other than those currently used. The advantages and disadvantages of using carbon fibers in an epoxy matrix are reviewed as well as current reflector fabrications technology and adjustment. The manufacturing sequence and coefficient of thermal expansion of carbon fiber/borosilicate glass composites is described. The construction of a parabolic reflector from this material and the assembling of both reflector and antenna are described. A 3M-aperture-diameter carbon/glass reflector that can be used as a subassembly for large reflectors is depicted. The deployment sequence for a 10.5M-aperture-diameter antenna, final reflector adjustment, and the deployment sequence for large reflectors are also illustrated
Force-induced rupture of a DNA duplex
The rupture of double-stranded DNA under stress is a key process in
biophysics and nanotechnology. In this article we consider the shear-induced
rupture of short DNA duplexes, a system that has been given new importance by
recently designed force sensors and nanotechnological devices. We argue that
rupture must be understood as an activated process, where the duplex state is
metastable and the strands will separate in a finite time that depends on the
duplex length and the force applied. Thus, the critical shearing force required
to rupture a duplex within a given experiment depends strongly on the time
scale of observation. We use simple models of DNA to demonstrate that this
approach naturally captures the experimentally observed dependence of the
critical force on duplex length for a given observation time. In particular,
the critical force is zero for the shortest duplexes, before rising sharply and
then plateauing in the long length limit. The prevailing approach, based on
identifying when the presence of each additional base pair within the duplex is
thermodynamically unfavorable rather than allowing for metastability, does not
predict a time-scale-dependent critical force and does not naturally
incorporate a critical force of zero for the shortest duplexes. Additionally,
motivated by a recently proposed force sensor, we investigate application of
stress to a duplex in a mixed mode that interpolates between shearing and
unzipping. As with pure shearing, the critical force depends on the time scale
of observation; at a fixed time scale and duplex length, the critical force
exhibits a sigmoidal dependence on the fraction of the duplex that is subject
to shearing.Comment: 10 pages, 6 figure
Coarse-grained simulations of DNA overstretching
We use a recently developed coarse-grained model to simulate the
overstretching of duplex DNA. Overstretching at 23C occurs at 74 pN in the
model, about 6-7 pN higher than the experimental value at equivalent salt
conditions. Furthermore, the model reproduces the temperature dependence of the
overstretching force well. The mechanism of overstretching is always
force-induced melting by unpeeling from the free ends. That we never see S-DNA
(overstretched duplex DNA), even though there is clear experimental evidence
for this mode of overstretching under certain conditions, suggests that S-DNA
is not simply an unstacked but hydrogen-bonded duplex, but instead probably has
a more exotic structure.Comment: 11 pages, 11 figure
DNA nanotweezers studied with a coarse-grained model of DNA
We introduce a coarse-grained rigid nucleotide model of DNA that reproduces
the basic thermodynamics of short strands: duplex hybridization,
single-stranded stacking and hairpin formation, and also captures the essential
structural properties of DNA: the helical pitch, persistence length and
torsional stiffness of double-stranded molecules, as well as the comparative
flexibility of unstacked single strands. We apply the model to calculate the
detailed free-energy landscape of one full cycle of DNA 'tweezers', a simple
machine driven by hybridization and strand displacement.Comment: 4 pages, 5 figure
Electromagnetic form factors of the baryon octet in the perturbative chiral quark model
We apply the perturbative chiral quark model at one loop to analyze the
electromagnetic form factors of the baryon octet. The analytic expressions for
baryon form factors, which are given in terms of fundamental parameters of
low-energy pion-nucleon physics (weak pion decay constant, axial nucleon
coupling, strong pion-nucleon form factor), and the numerical results for
baryon magnetic moments, charge and magnetic radii are presented. Our results
are in good agreement with experimental data.Comment: 15 pages, 6 figures, to be published in Eur. Phys. J.
The effect of topology on the structure and free energy landscape of DNA kissing complexes
We use a recently developed coarse-grained model for DNA to study kissing
complexes formed by hybridization of complementary hairpin loops. The binding
of the loops is topologically constrained because their linking number must
remain constant. By studying systems with linking numbers -1, 0 or 1 we show
that the average number of interstrand base pairs is larger when the topology
is more favourable for the right-handed wrapping of strands around each other.
The thermodynamic stability of the kissing complex also decreases when the
linking number changes from -1 to 0 to 1. The structures of the kissing
complexes typically involve two intermolecular helices that coaxially stack
with the hairpin stems at a parallel four-way junction
The self-assembly of DNA Holliday junctions studied with a minimal model
In this paper, we explore the feasibility of using coarse-grained models to
simulate the self-assembly of DNA nanostructures. We introduce a simple model
of DNA where each nucleotide is represented by two interaction sites
corresponding to the phosphate-sugar backbone and the base. Using this model,
we are able to simulate the self-assembly of both DNA duplexes and Holliday
junctions from single-stranded DNA. We find that assembly is most successful in
the temperature window below the melting temperatures of the target structure
and above the melting temperature of misbonded aggregates. Furthermore, in the
case of the Holliday junction, we show how a hierarchical assembly mechanism
reduces the possibility of becoming trapped in misbonded configurations. The
model is also able to reproduce the relative melting temperatures of different
structures accurately, and allows strand displacement to occur.Comment: 13 pages, 14 figure
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